This invention relates generally to the zeolite catalyzed conversion of one or more C.sub.1 -C.sub.4 oxygenates such as methanol to hydrocarbons boiling within the gasoline range and, more particularly, to the treatment of all or a portion of a durene-containing effluent or fraction resulting from said conversion in order to reduce its durene content.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
The conversion of C.sub.1 -C.sub.4 oxygenates such as methanol to gasoline is an important area of technology which has the potential of becoming even more important as the supply of crude oil is diminished and/or increased in price. Particularly advantageous catalysts which are utilized in the conversion of C.sub.1 -C.sub.4 oxygenates to gasoline are a special class of porous, acidic crystalline silicate, or zeolite, catalysts of which HZSM-5 is the most preferred member. There are many patents and publications which describe the conversion of oxygenates to gasoline over the zeolites, among them U.S. Pat. Nos. 3,894,102; 3,894,104; 3,899,544; 3,904,916; 3,911,041; 3,931,349; and, 3,969,426, the disclosures of which are incorporated by reference herein.
One particular problem residing in the conversion of C.sub.1 -C.sub.4 oxygenates to gasoline over such zeolites, e.g. ZSM-5, is that durene is produced in amounts higher than that expected from C.sub.10 aromatic equilibrium distributions. Once an aromatic ring is formed in the presence of unreacted methanol, alkylation to tetramethylbenzenes occurs rapidly but the smaller, higher melting durene molecule (1,2,4,5-tetramethylbenzene, melting point 175.degree. F.) diffuses out of the ZSM-5 pores much more rapidly than isodurene (1,2,3,5-tetramethylbenzene). Durene is an undesirable high boiling aromatic with a tendency to crystallize out at low temperatures thus forming a solid film on heat exchanger tubes and process lines, often breaking up into a sludge which plugs the lines through which it is passed.
Various proposals have been advanced for dealing with durene which is produced in the catalytic conversion of C.sub.1 -C.sub.4 oxygenates to gasoline, the proposals generally falling into two broad categories. One approach to the problem is to vary the conversion conditions so that durene is either not formed at all or, at most, is formed only in small amounts. An approach of this type is represented by U.S. Pat. No. 4,025,576 which discloses that durene formation is reduced if methanol is first converted to olefins in a first stage and the olefins are thereafter converted to gasoline range hydrocarbons in a second stage.
The second approach with regard to durene control makes no attempt to control the amount of durene which is formed in the conversion of C.sub.1 -C.sub.4 oxygenates to gasoline but seeks to convert at least some of the durene to other products. This is the approach taken in the processes disclosed in aforementioned U.S. Pat. Nos. 3,969,426 and 4,347,397.
As disclosed in U.S. Pat. No. 3,969,426, the durene produced in a methanol to gasoline conversion process is diminished by reacting a durene-containing stream with one or more low boiling aromatics, e.g., benzene, in order to transalkylate, and thereby reduce, the durene content of the stream. A disadvantage of this process lies in the fact that in the ZSM-5 catalyzed conversion of methanol to gasoline such as described in this patent, little benzene is produced. Therefore, in order to carry out the durene-reduction process of U.S. Pat. No. 3,969,426, an external source of benzene must be made available for reaction with the durene-containing component of the methanol to gasoline conversion process effluent. This drawback is obviated by the process of U.S. Pat. No. 4,347,397 which does not require benzene (or other transalkylating aromatic) and, in fact, is preferably conducted in the substantial absence of benzene, i.e., with no more than about 5 wt.%, and preferably no more than about 1 wt.% benzene, being present.
In accordance with the process disclosed in aforesaid U.S. Pat. No. 4,347,397, the total durene-containing effluent from a methanol to gasoline conversion process or a durene-containing bottoms fraction thereof (obtained from the total gasoline fraction by topping off at least a light olefinic fraction) is treated by isomerization at elevated temperature and pressure over known and conventional isomerization catalysts including amorphous catalysts such as silica-alumina, silica-magnesia, silica-zirconia, silica-alumina-magnesia, silica-alumina-zirconia, metal phosphates, etc., as well as crystalline aluminosilicate zeolites such as zeolite X, Y, ZSM-4, Zeolite Beta, ZSM-11, ZSM-12, etc., to isomerize durene to other tetramethylbenzene, 1,2,3,5-tetramethylbenzene but some 1,2,3,4-tetramethylbenzene as well, both of which have lower melting points than durene.